This invention relates to substantially spherical magneto-plumbite ferrite (barium or strontium ferrite) particles and methods for their production, and to magnetic recording media formed therewith.
Magnetic recording media are employed in a wide variety of applications, including identity cards, credit cards, banking cards, parking permits, hotel key cards, tollgate cards, data tapes, and floppy disks. In these and similar applications, it is desirable to provide a recording medium that both minimizes unintended erasure and maximizes storage capacity (bit density).
A magnetic recording media requires adequately high coercivity (a measure of magnetic field strength of a magnetized substance and of its resistance to demagnetization) to minimize accidental loss of stored data. In magnetic stripe cards, for example, accidental erasures associated with low coercivity of the recording medium account for 60% of card failures.
A magnetic media also preferably has relatively low magnetic interaction (a measure of the degree of magnetic interaction of one point on the media with adjacent areas on the media). Lower magnetic interaction allows increased bit density, providing increased data storage capacity in the same space.
Increases in storage capacity could provide extended capabilities in devices employing magnetic media. In the case of magnetic stripe cards, because a standard banking credit card has only 140 bytes of storage, a typical consumer uses several separate magnetic stripe cards. Larger storage capacity would allow the combination of several cards into one multifunctional magnetic card. New functions not currently performed by magnetic cards could even be added to such a card.
Magneto-plumbite ferrite particles have been used in magnetic recording media. Magneto-plumbite ferrite particles have the advantage of relatively high coercivity (e.g., anywhere from 300 to more than 3500 Oe), and have shown a corresponding greater resistance to erasure than media using typical acicular (needle-like) metal particles.
But magneto-plumbite ferrite particles have the disadvantage of tending to group together in clumps or stacks during general production, processing, and handling of the particles. Additionally, in the production of a magnetic layer employing magneto-plumbite ferrite particles, the still-wet layer is subjected to a magnetic field to orient the particles. This magnetic orientation process significantly increases particle grouping of magneto-plumbite ferrite particles. Groups of particles tends to act magnetically as a single particle, resulting in uneven and larger-than-desired effective particle sizes and increased magnetic interaction. Such groups of particles produce uneven magnetic properties in a magnetic media, resulting in high media noise and correspondingly lower maximum bit densities. To increase the storage capacity of a magnetic media, the density of such defects must be decreased in order to maintain an adequate media signal-to-noise ratio.
Grouping of particles during particle preparation can result in a small amount of strongly bonded clumps or stacks (agglomerates) remaining in the prepared powder. Such agglomerates do not disperse during the milling process for preparation of a magnetic paint. After the milling process, these non-dispersed particles clog filter pores during the filtering process, decreasing filter efficiency. The non-dispersed particles can also contribute to defects such as pinhole, stain, and rough surface on the top surface of the magnetic layer. These defects cause various problems, such as media noise (including dropout and signal spike), collision between head and media, etc.
At CardTech/SecurTech in 1996, Eastman Kodak Company announced the Intelligent Magnetics card system, having comparable capabilities to a chip (or smart) card but at much less cost. The MR (magneto resistive) head used for high-density storage applications such as the Intelligent Magnetics card system is much more sensitive than inductive heads used in current card systems. Consequently, a magnetic card for such a system requires a small concentration of magnetic particles uniformly dispersed throughout the whole card. If there are clumps or stacks of non-dispersed magnetic particles in a card, a magnetic field higher than that of the surrounding magnetic particles will be generated, resulting in a signal spike. In a card designed for high storage capacity, uniformity of dispersion is critical to obtaining a stable signal from the card.
For all of the above reasons, it is desirable to minimize particle grouping and maximize the uniformity of dispersion of magneto-plumbite ferrite particles for use in magnetic recording media.
The present invention introduces substantially spherical magneto-plumbite ferrite particles, processes for the production of such particles, and magnetic media employing such particles. The particles of the present invention provide improved uniformity of dispersion and decreased magnetic interaction between particles in magnetic recording media compared to previous forms of magneto-plumbite ferrite particles. The process for the preparation of the substantially spherical magneto-plumbite ferrite particles results in a narrow particle size distribution, controllable coercivity (Hc), and excellent dispersion stability with reduced particle stacking or clumping in the organic solvents and binders used to produce magnetic recording media.
Particle clumping and stacking in media is influenced by several factors, including properties of the particles such as particle shape, diameter, and aspect ratio, and other factors such as viscosity and composition of the magnetic paint, the applied magnetic field used for particle orientation, and milling conditions during processing of the particles. However, it has been found that a principle cause of particle stacking in magneto-plumbite ferrite particles is the platelet shape of typical magneto-plumbite ferrite particles. Typical magneto-plumbite ferrite particles have a flat, hexagonal shape like a six-sided plate. The mean aspect ratio of such xe2x80x9cplateletxe2x80x9d magneto-plumbite ferrite particles, i.e., the mean ratio of the longest dimension (diameter) to the shortest dimension (thickness), is typically not less than about 3:1, and can be significantly higher. Particles having higher aspect ratios tend to stack or clump together more than those having lower aspect ratios.
Acicular (needle-like) magneto-plumbite ferrite particles have also been reported. Acicular particles, with aspect ratios often as great as 10:1 or more, are also difficult to disperse evenly.
The substantially spherical shape and the associated low aspect ratio (approaching 1:1) of the particles of the present invention significantly reduces the tendency of the particles to form stacks or clumps, as compared to previous magneto-plumbite ferrite particles. Preventing particles from clumping or stacking decreases the degree of magnetic interaction and improves dispersion stability in a magnetic paint or other magnetic medium. The even dispersion that is achieved with the use of the particles of the present invention allows the creation of higher-density magnetic media, allowing more information to be stored within the same amount of space. The increased storage capacity allows the creation of such devices as magnetic cards that function like smart cards, and higher capacity magnetic storage disks and tapes for digital data handling and storage.
The chemical formula of the particles of the present invention is as follows:
AOxc2x7n(Fe2xe2x88x92xMxO3)xe2x80x83xe2x80x83(1)
where A is Ba, Sr, or mixtures thereof, n is within the range of from about 5.0 to about 6.0, and M is more than one of the group of substitution elements Co, Zn, Ni, Mn, Al, Ti, Sn, Si, Nb, and Ta, and x is within the range of from about 0 to about 0.35. The particles are substantially spherical, having a mean aspect ratio generally less than about 2:1, desirably less than about 1.5:1, most desirably less than about 1:25:1.
The particles of the present invention are prepared by mixing ultra-fine (e.g., about 0.05 to about 1.2 xcexcm diameter) substantially spherical iron-based oxide and/or hydroxide particles, such as magnetite (Fe3O4), maghemite (xcex3-Fe2O3), hematite (xcex1-Fe2O3), spinel ferrite ((Co, Ni, Zn, Mn, Ti, Si)xFe3xe2x88x92xO4), and/or iron hydroxide (FeOOH) with a source of Barium and/or Strontium, such as colloidal barium or strontium carbonate (BaCO3 or SrCO3), with small amounts of a byproduct, such as sodium or potassium chloride (NaCl or KCl), hydroxide (NaOH or KOH), or nitrate (NaNO3 or KNO3) functioning as a flux to lower the calcination temperature.
The solid phase of the dispersion is then filtered off, dried, and calcined at a temperature sufficiently high and for a time sufficiently long to produce the desired magneto-plumbite ferrite, yet sufficiently low and sufficiently short to allow the particles to substantially retain the shape of the iron-based oxide/hydroxide precursor particles. Temperatures generally not more than about 900xc2x0 C. allow the particles to substantially retain the shape of the precursor particles, desirably in the range of about 730xc2x0 C. to about 870xc2x0 C. In the working examples described below, particles according to the present invention were produced at calcination temperatures in the range of about 780xc2x0 C. to about 810xc2x0 C., at times from about 1 to about 2 hours.
The substantially spherical magneto-plumbite ferrite particles of the present invention prepared as described herein have a coercivity in the range of from about 1000 Oe to about 5500 Oe, high saturation magnetization above about 35 emu/g, and narrow coercivity distribution (evidenced by a measured switching field distribution (SFD) of 0.40 for spherical barium ferrite particles compared to 0.60 for platelet barium ferrite particles in an otherwise identically-prepared magnetic medium), while maintaining excellent dispersibility and reducing or eliminating particle stacking. These properties are well suited to magnetic recording applications. Hand-coated magnetic tapes prepared with the particles of the present invention have shown good magnetic properties, as set forth in the examples below. Thus the substantially spherical magneto-plumbite ferrite particles of the present invention may suitably be used as the magnetic powder material of magnetic recording tape, and would also find application in such media as magnetic stripe cards, floppy disks, etc., particularly for high-capacity storage applications.
The process of the present invention economically allows the mass production on an industrial scale of the spherical magneto-plumbite ferrite particles having the desired properties mentioned above. Magnetic storage media may be prepared from the particles of the present invention by conventional means, such as mixing the particles in a magnetic paint, then coating the paint on a substrate such as a card, a tape, or a disk, or such as mixing the particles directly into a polymer or the like, then forming a card, a tape or a disk of the polymer.